Method of manufacturing a radome

ABSTRACT

A radome and the method of manufacture thereof are presented wherein the radome is constructed from a series of rings of fiber reinforced polytetraflouroethylene. The rings are machined from a cold molded PTFE-fiber composite billet and then loaded into a mold cavity. The rings are then subjected to heat and pressure to sinter them together. The resulting structure is machined into its final shape.

BACKGROUND OF THE INVENTION

This invention is directed to the fabrication of unitary structures,such as missile radomes, from fiber reinforced plastic material. Moreparticularly, this invention relates to radomes which are resistant toablation and rain erosion, particularly at high operating speeds such asMach 4 or higher.

Ceramic radomes are typically used for missiles intended to operate atspeeds of Mach 4 or higher. These ceramic radomes have been found to bemarginal in performance due to fragility, susceptibility to thermalshock, high thermal conductivity, high rates of rain impact damage. Adefinite need exists for a workable alternative to ceramic radomes.

Radomes made from polymeric composite materials have been suggested as apossible alternative to ceramic radomes. Polytetrafluoroethylene,hereinafter PTFE, is one such polymeric material which might be suitablefor radome applications. However, "neat" or simple filled PTFE does notpossess the requisite characteristics, uniformity of erosion andablation for example, for use in the demanding environment of a missileradome. Tests have shown that fiber filled PTFE; i.e., a PTFE composite;would have those characteristics dictated by radome and similar usage.

Prior to the present invention it has been a practical impossibility tofabricate a radome from a PTFE-fiber composite. The production of asolid block of a PTFE composite of sufficient size to permit machining aradome therefrom is not feasible due to the virtual impossibility ofheating such a large block through the crystalline melt point andsubsequently cooling through the recrystallization point with enoughuniformity of temperature to avoid fissures and damage from thermalstress. Further, even if the temperature gradient and thermal stressproblems could be avoided, an extremely long heating and cooling cycle(perhaps on the order of several weeks) would be required, and that longcycle time would result in thermal degradation. Other approaches, suchas flowing a sheet of PTFE composite material to form a radome shape orlaminating a series of rings or discs cut from such sheet material allinvolve substantial technical or cost problems which have previouslyprecluded the use of such material and techniques.

SUMMARY OF THE INVENTION

The present invention overcomes the above discussed and other problemsand provides a large and complex shaped unitary structure formed from aPTFE-fiber composite and suitable for radome use. In accordance with thepresent invention, discs or rings are molded from a PTFE-fibercomposite. The fibers are oriented during molding so as to bepredominantly in the direction perpendicular to the axis of the disc andthus, when a radome is to be formed, also perpendicular to the axis ofthe radome. The discs are then machined to form a series of preforms orsegments of desired sizes and shapes. These segments are stacked in amold with abutting surfaces of adjacent segments arranged generallyperpendicular to the wall of the mold at the joints. Thisperpendicularity is achieved by forming "beveled" faces on the segments.These abutting "beveled" faces prevent radial slippage between segmentsduring subsequent processing steps. The cavity of the mold is shaped tohave the general shape of the desired outer surface of the finishedradome or other part to be formed. The segments in the mold are thensubjected to axial pressure and are sintered in a heating cycle whereinthe polymer is taken through the crystalline melt point, heat soakedabove the melt point to effect diffusion across the joints and thus toeffect thermal bonding across the boundaries of the adjacent rings,slowly cooled to and through the recrystallization point, and thencooled back to room temperature. The bonded structure is then removedfrom the mold and the inner and outer surfaces are machined to thedesired final contour of the radome or other part.

Radomes made in accordance with the present invention are fiberreinforced shapes with the fibers nonisotropically oriented in apreferred direction to reduce the ablation and rain erosion rates andmake the ablation and rain erosion more even as compared to "neat" orunfilled PTFE. The product is a fully bonded laminate structure whereinthe bonding is effected without any extraneous adhesive or other bondingmedium. Also, the end product is relatively economical to producebecause the manufacturing process conserves and reduces the amount ofmaterial necessary to produce the end product. Also, the end product,although being a laminate, has extremely strong bonding between rings asa result of the manufacturing step wherein the rings are arranged withabutting surfaces perpendicular to the wall of the mold duringprocessing.

Accordingly, one object of the present invention is to provide a noveland improved molded part of fiber reinforced PTFE.

Another object of the present invention is to provide a novel andimproved radome structure and method of manufacture thereof.

Still another object of the present invention is to provide a novel andimproved radome structure and method of manufacture thereof wherein theradome is formed of a PTFE-fiber composite.

Other objects and advantages of the present invention will be apparentto and understood by those skilled in the art from the followingdetailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like elements are numbered alikein the several FIGURES:

FIG. 1 schematically shows a series of molded PTFE-fiber composite discsand, in cross-section preform segment shapes which will be formedtherefrom;

FIG. 2 is a schematic cross-sectional elevation view of a mold structurewith segments assembled in the mold ready for sintering;

FIG. 3 shows a side elevation view, partly in section, of a finishedradome; and

FIG. 4 is a flow diagram of the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the drawing, and particularly to FIGS. 1, 2 and 4,the first step 20 in the practice of the present invention comprisesformation of a series of discs 22, 24, 26 and 28. These discs arecomprised of from 95 to 50 parts by weight of PTFE; i.e., TEFLON 7A; andfrom 5 to 50 parts by weight of reinforcing fibers. The reinforcingfibers may be comprised of a ceramic material, microfiberglass or othersimilar materials. Thus, by way of example, the fibers may compriseJohns-Mannvill Company Type 104 E microfiberglass or "Fiberfrax"aluminum silicate fibers available from Carborundum Corporation. Thefibers, which are inorganic, will typically range in diameter from 0.05to 10 micrometers and will preferably have an aspect ratio of at least30.

The powder from which the discs 22, 24, 26 and 28 are formed ispreblended, screened to insure against lumps, are milled and coldmolded. It is very important that the discs 22, 24, 26 and 28, and thusthe preforms which are to be cut therefrom be characterized by uniformfiber dispersion and uniform density. This is achieved by compacting thepowder in a cold molding step with the application of direct linearpressure. This manner of forming the discs results in the majority ofthe fibers assuming an orientation wherein they are generallyperpendicular to the direction of molding compression.

As indicated at step 32 in FIG. 4, the discs are machined to obtain aseries of preform rings 1-13 shaped as shown in FIG. 1. These preformsare numbered consecutively in the order in which they will be placed ina mold as will be described below in the discussion of FIG. 2. As willbe described in greater detail below, the preforms are sized and shapedso that, when stacked in the mold, the abutting surfaces will be inintimate contact. Additionally, the angles at which these abuttingpreform surfaces are cut; i.e., the slopes of the end faces of therings; are chosen such that these surfaces are generally perpendicularto the wall of the mold in which the preforms are stacked to form agenerally conical configuration of radome shape. As indicated in FIG. 1,several rings can be machined from each of the cold molded discs. As maybe seen from FIGS. 1 and 2, all of the preforms with the exception ofthe end segment 1 are annular. The end segment 1 is machined to conformto the shape of the bottom inner surface of the mold cavity in which thepreforms are to be placed.

In one reduction to practice of the invention the preform rings 1-13were lathe cut from the discs 22-28. This was accomplished by firstfacing off both sides of each disc with the second face being cut to aconstant compound feed setting. Next, the first side of each disc wascut in such a manner that the cutting penetrated half-way through thethickness of the disc. Subsequently, cuts were made from the second sideof each disc and deeply enough to get by the corners to subsequently bemade by the beveled faces. During this second cutting procedure,sufficient material was left between the rings for support. Face angleswere then cut on the first disc sides, starting at the outer edge ofeach ring, to form first beveled faces. Next, face cuts are preformed onthe second sides, starting at the inner edge of each ring, to form thesecond beveled faces. Finally, cutting from the second side, the ringswere parted. In summary, the second step 32 in the practice of thepresent invention comprises the machining of beveled sided, flat facedrings from cold molded discs of fiber reinforced PTFE molding compound.Although other machining techniques may be employed, lathe machining hasbeen found to be a practical procedure for producing the preform ringsfrom the molded billets 22, 24, 26 and 28.

Referring to FIG. 2, and as indicated at step 34 in FIG. 4, the machinedpreform rings are loaded into the cavity of a mold 40 as shown. In onereduction to practice of the invention mold 40 comprised a block ofaluminum machined so as to have an inside contour which matched thefinal desired shape of a radome. The mold cavity was designed to allowfor axial expansion and contraction of the molded part and also so as toallow for radial shrinkage. The preform rings are to be sintered into aunitary structure and, in order to avoid fissures or other thermallyinduced defects, means must be provided to uniformly heat mold 40. Inthe disclosed embodiment, the heating means comprises four resistancetype heaters 42 in the form of bands wrapped around the exterior of mold40. It has also been found necessary, in order to prevent air fromattacking the polymer in the region of the interfaces between the rings,to provide for the hermetic sealing of the mold cavity with respect tothe ambient environment. Accordingly, a clamping ring 44 is providedabout the upper periphery of mold 40. Ring 44 is employed to clamp anedge of an aluminum foil envelope 46 to the upper side of mold 40.Envelope 46 is provided with a hose connector, as indicated at 48,whereby the interior of the envelope and mold may be coupled to a sourceof pure dry nitrogen.

In the embodiment being disclosed, the mold 40 is supported on a blockof thermal insulating material 49 which, in turn, rests on a movableplate 50. Plate 50 is connected, via a push rod 52 to a hydrauliccylinder coupled to a controlled pressure source whereby axial pressuremay be applied to the part being formed during the sintering operation.The push rod 52 and plate 50 urge mold 40 upwardly toward a fixed plate54 from which, by means of a block 56, a further thermal insulator 58 issupported. During the loading of the mold cavity, prior to installationof aluminum foil envelope 46, a retainer assembly is installed in themold cavity. This retainer assembly includes an aluminum pusher disc 60which is supported on a ring 62 which extends downwardly from a furtherblock of thermal insulating material 64. A portion of the foil envelope46 is sandwiched between thermal insulating members 58 and 64. Thepusher disc 60 is provided with an aperture 66, or a plurality ofapertures, to permit nitrogen to flow to the interior of the moldcavity. Disc 60 is also provided with an annular cut-out which forms ashoulder extending about the lower periphery thereof. This shoulderengages the uppermost ring 13 as shown in FIG. 2. The disc 60 preventsthe stack of preform rings from sagging inwardly during the sinteringstep and, in combination with the hydraulic cylinder, applies axialforce to the stack.

After the mold has been loaded as indicated at step 36 in FIG. 4, thesintering cycle is begun. Simultaneously with energization of heaters42, the flow of nitrogen is started. The air is diffused out of the moldcavity by maintaining nitrogen above disc 60. Restated, by the time thePTFE composite reaches a temperature above 300° C., where attack byoxygen could occur, all air will have been swept out of the mold by thenitrogen. In accordance with the present invention, a long sinteringcycle is employed to achieve temperature uniformity during the meltphase, when the polymer is in the crystalline melt stage, and to permitslow squeeze flow to maintain material conformity to the mold shapeduring the thermal expansion which occurs as the material passes throughthe crystalline melt point. It is also to be noted that, as representedin FIG. 2, adequate mold cavity space above the preform ring stack isrequired so that, during peak expansion, the part being formed does notextend above the top surface of the mold and seal off the nitrogen flowwhich prevents air from entering the aluminum foil seal. Axial force isapplied to the preform rings in the mold during the sintering step. Thisapplied force is maintained by monitoring and controlling the pressurewithin the hydraulic cylinder connected to push rod 52. It has beenfound adequate to maintain the hydraulic pressure constant whereby,because of expansion and contraction of the work, the applied force willvary. The applied force causes the abutting beveled faces of themachined preform segments to maintain conformance and contact with oneanother through the various stages of the sintering cycle includingespecially the expansion during crystalline melt and the soaking periodabove the crystalline melt temperature.

As noted above, in accordance with the present invention a longsintering cycle is employed. An important characteristic of thissintering cycle is that the transition temperature of the polymer bepassed through during a slow rise segment of the cycle. Through the useof suitable instrumentation, not shown, the heating may be programmedand subsequently controlled pursuant to a multistep schedule. In onereduction to practice, the preform rings were comprised of 85% by weightPTFE, 15% by weight microglass fibers having an average diameter ofabout 0.2 micrometers and an average length of about 2 millimeters andthe rings ranged from 13 to 22 millimeters in thickness. The thicknesswas measured perpendicular to the mold walls and was a function of moldcavity diameter. Also during sintering, an axial force in the range of3.35 to 4.13 kilonewtons was applied to mold 40. The sintering cycle forthe example being described was controlled as follows:

    ______________________________________                                        Hours from                                                                    Start     0      4       24    32    46    50                                 Mold Tempera-                                                                 ture (°C.)                                                                       23     275     370   370   275   100                                ______________________________________                                    

It is to be noted that the rate of heating up through the melt point ofthe polymer; i.e., as the temperature rises from 275° C. to 370° C.; isof very long duration when compared to prior portion of the heatingcycle. It is further to be noted that the maximum temperature, whichoccurs in the 24 to 32 hour period of the cycle, will be in the range of380° to 395° C. which is well below the temperature at whichdecomposition of PTFE will begin. Bearing in mind that virgin PTFE has amelt point in the range of 333° to 338° C., the present inventioncontemplates a heat soak above the melt point for a long period of timeto insure a thermal "knit" across the boundaries of the preform rings.This heat soak above the melt point will typically be of from 4 to 9hours duration. The polymer is initially basically crystalline in form.When heated above the melt point, the polymer becomes a rubberyamorphous material and the polymer chains will, to some extent, diffuseacross joints whereby the individual rings will, partly as a consequenceof the applied axial pressure, join to form a unitary structure. It isalso to be noted that when the polymer reaches melt point, it willexpand dramatically but this expansion will occur, with a smalltemperature rise, over a long period of time. If the abutting sidesurfaces of the preform rings were not "beveled"; i.e., perpendicular tothe wall of the mold cavity; slippage between rings would be likely tooccur during this expansion.

It has been also found to be desirable to slowly cool the formed part toand through the recrystallization point, which will be in the range offrom 315° to 320° C., in the interest of solidification of the entirepart at the same time. This manner of cooling is in the interest ofminimizing shrink stresses. The part will shrink more in the lengthdirection than in the radial direction as a result of theabove-described orientation of the fibers. However, there is sufficientradial shrinkage that the part will move away from the inner wall of themold except, of course, at the very bottom. It is for this reason thatthe tip ring 1 is formed in such a manner that it will not bind with themold cavity.

Subsequent to cooling, the radome blank, or other part, will be removedfrom mold 40 and machined, as indicated at step 66 in FIG. 4, to thedesired final shape as shown at 68 in FIG. 3.

While the above discussion has been limited to PTFE and fibercomposites, other fluoropolymers may be added to the composites for thepurpose of modifying the processing requirements for attaining certaindesirable characteristics. Typically such additives will possess lowermelting temperatures, lower melt viscosity, better ability to wet fiberor filler surfaces, and to close voids in preforms. Fluoropolymeradditives that could be used include, but are not limited to copolymersof tetrafluoroethylene and hexafluoropropene. Also, Teflon PFA, apolymer with perfluoroalhoxy side groups, could be added to the fiberreinforced composites used in the practice of the present invention.

While a preferred embodiment has been shown and described, varioussubstitutions and modifications may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustration and not limitation:

What is claimed is:
 1. A method for the production of complex shapesfrom fiber reinforced polymeric material comprising the steps of:forminga mixture of polymeric material in powder form and reinforcing fibers;cold molding the mixture to form a fiber-polymer composite billetwherein the majority of the fibers are orientated in a desired direction; machining preform segments of the complex shape from the billet, thesegments being formed to have planar abutting surfaces; stacking thesegments in a mold; and heating the mold while applying force to thesegments to sinter the polymer composite and effect bonding betweenadjacent segments.
 2. The method of claim 1 wherein the mold has acavity with the desired complex shape and wherein the step of machiningcomprises:cutting each of the segments to have a first surface areawhich conforms to a portion of the mold cavity wall; and cutting thesurfaces of the segments which are to abutt other segments such thatsuch abutting surfaces will be generally perpendicular to the moldcavity wall portion to which the segment first surface area conforms. 3.The method of claim 2 wherein the polymeric material is a fluoropolymerand wherein the step of heating includes:heating the mold at a firstrate to a first temperature below the crystalline melt point of thepolymer; heating the mold at a second rate from said first temperatureto a second temperature above the crystalline melt point of the polymerbut below the decomposition temperature of the polymer, the temperaturerise during heating at said second rate being slower than during heatingat said first rate; maintaining the mold temperature above thecrystalline melt point of the polymer for a preselected time; andcooling the mold to room temperature.
 4. The method of claim 3 whereinthe complex shape is at least in part tubular and wherein at least someof the segments are in the general form of rings.
 5. The method of claim4 wherein the complex shape is a radome blank having an axis and theforce applied is along said axis, the abutting surfaces of the segmentsbeing oriented at angles other than 90° with respect to said axiswhereby said abutting surfaces cooperate to resist relative radialmotion between adjacent segments.
 6. The method of claim 5 wherein thestep of cold molding includes the application of direct linear pressureto form a disc wherein the majority of fibers are perpendicular to thedirection of molding compression of the disc.
 7. The method of claim 6wherein the step of forming the mixture comprises:mixing from 95 to 50parts by weight of polytetrafluoroethylene with from 5 to 50 parts byweight of inorganic microfibers.
 8. The method of claim 7 wherein theabutting segment surfaces are in intimate contact and slope upwardly inthe direction of said axis and wherein said segments are capable ofaxial movement and the uppermost segment is engaged by a support toprevent radial movement thereof.
 9. The method of claim 1 furtherincluding the step of:milling the sintered polymer composite segments toform the final complex shape.